1. Field of the Invention
This invention relates to a magneto-optical recording medium for use as a recording medium for quick information storage, image recording or the like, and a method of making the magneto-optical recording medium. This invention particularly relates to a magneto-optical recording medium in which a magneto-optical recording layer is protected from oxidation, and a method of making the magneto-optical recording medium.
2. Description of the Prior Art
Optical recording media have various advantages such as that they can record information at a high density, have a large capacity, and need not be contacted by a read/write head. Among such optical recording media, the magneto-optical recording medium has attracted particular attention for enabling easy erasing and rerecording.
The magneto-optical recording medium is composed of a magnetic material as a recording medium material, and records information based on changes in magnetization of the magnetic material. For example, an amorphous rare earth metal-transition metal alloy composed mainly of a combination of rare earth metal (RE) such as Gd, Tb or Dy with a transition metal such as Fe, Co or Ni is used as the magnetic material. The amorphous rare earth metal-transition metal alloy is used in a layer form as a magneto-optical recording layer.
The rare earth metal-transition metal alloy is readily oxidized. When the rare earth metal-transition metal alloy used as the magnetic material is oxidized, magnetic characteristics such as coercive force thereof deteriorate and efficient magneto-optical recording and reproduction may become impossible. Therefore, for practical use of the magneto-optical recording medium, it is necessary to prevent oxidation of the rare earth metal-transition metal alloy (i.e. the magneto-optical recording layer).
As disclosed in, for example, Japanese Unexamined Patent Publication No. 60(1985)-182535, in order to prevent oxidation of the magneto-optical recording layer, it has been proposed to cover the overall surface of the portion of the magneto-optical recording layer which contacts the ambient air with a protective layer formed of SiO.sub.2 or SiN.sub.x, where x designates a number within the range of 0.5 to 1.33. However, the effect of such a protective layer is merely to give the magneto-optical recording layer some degree of shielding from the ambient air by a substance which does not react with oxygen, and there is no positive effect of trapping oxygen. Therefore, with the proposed technique, it is not always possible to completely prevent the magneto-optical recording layer from oxidation. Particularly, the oxidation prevention effect is insufficient at the edge faces of the magneto-optical recording layer which are shielded from the ambient air only by a thin protective layer. Accordingly, oxidation progresses from the edge faces of the magneto-optical recording layer, and deterioration of the recording and reproduction characteristics, and deterioration of the appearance by cracking, pitting, corrosion, pinholes or the like occur.
Similar art of protecting the overall surface including the edge surface of the magneto-optical recording layer with a protective layer is disclosed in other Japanese patent and utility model publications such as Japanese Unexamined Patent Publication Nos. 57(1982)-27494, 58(1983)-60442, 60(1985)-182535, 60(1985)-258747, 62(1987)-40651, 62(1987)-164244, 62(1987)-167635, 62(1987)-167636, and Japanese Unexamined Utility Model Publication No. 62(1987)-113423.